Types of pollination: cross-pollination, self-pollination & more

There are plenty of ways for plants to pollinate themselves, be it with wind or insects. Read on to find out exactly how plants are able to reproduce.

Now, what was that about the birds and the bees? Long after that fateful lesson in Biology class, it’s easy to forget exactly how plants are able to reproduce. Here’s a little refresher on pollinators, animal flowering, self- and cross-pollination, and more.

What is pollination?

Pollination is the process of transferring pollen to the stigma or ovule of a plant. Here, as with human reproduction, two halves of genetic material come together: one from the male pollen, the other from the female ovary. If all goes well, pollen moves through the plant’s pollen tube to reach the pistil of the flower. After several days, it will reach the embryo sac. Here, the pollen fuses with the female ovary, and the genetic material is finally combined.

What are the different types of pollination?

When it comes to pollination fertilising the plant, there are two types: self-pollination and cross-pollination.

Self-pollination

Self-pollinators that reproduce with themselves. They combine their own ovary with their own pollen to produce seed plants. As such, a few individual plants can produce an entire colony relatively quickly. Pioneer plants are often self-pollinators. Examples of self-pollination include: snowdrops (Galanthus nivalis), barley (Hordeum vulgare), beans (Phaseolus vulgaris), and peas (Pisum sativum).

Cross-pollination

Cross-pollinators cannot fertilise themselves. In order to reproduce, the pollen and ovaries must come from different individuals of the same species. Cross-pollination encourages genetic variability, meaning the plants can adapt to changes in the environment more readily.

Cross-pollination occurs in: primroses (Primula), loosestrife (Lythrum salicaria), wood sorrel (Oxalidaceae), irises (Iris), sage (Salvia), and maize (Zea mays).

Some plants tend to only self-pollinate. Others cross-pollinate. But some can do both! Cross-pollination is by far most common, and in fact, most plants have evolved to prevent self-pollination because cross-pollination leads to adaptability. However, whichever type of pollination a plant uses says nothing about whether it is pollinated by insects, the wind, bats, birds or even water.

Expert tip: Several evolutionary mechanisms prevent self-pollination. These include: separate flowering times for the male and female flowers, as is the case for the hazelnut (Corylus avellana); structuring the flower so that pollinating insects will graze foreign pollen onto the plant’s stigma before collecting its own pollen; enzymatic self-sterility, where the pollen is either prevented from germinating or specialised enzymes stop the pollen tube in the pistil before it reaches the ovary.

Insect pollination

Many plants depend on insects for pollination. No prizes for guessing what this is called: “insect pollination”. All insect-pollinating plants are brightly coloured, strongly scented or otherwise characterised with features that entice insects. Incidentally, many plants are specialised for “their” pollinators – and the same applies for the insects. Flower shape and depth, nectar composition, the insects’ flight height and distance, the time of flowering and hatching of insect larvae are all precisely coordinated by the plants to optimise insect pollination. You are likely familiar with bee pollination. But butterflies, flies, moths and many insects are just as important.

Examples of insect pollinators include: fruit trees such as apple (Malus), pear (Pyrus) and cherry (Prunus), lunkweed (Pulmonaria), arum (Arum), lime/linden (Tilia), chestnut (Aesculus), and meadow clover (Trifolium pratense), to name just a few!

Wind pollination

Wind is considered the original pollinator. In primeval forests, pollen was carried from plant to plant with wind alone. Wind pollinators can be recognised by their long, hanging catkins. The pollen in these catkins resembles dust, and is blown to the inconspicuous female flowers. These female flowers are often difficult to spot. They often lack petals or decorative features, leaving only stigma branches on which the pollen lands.

Examples of wind pollination: hazelnut (Corylus avellana), willow (Salix), alder (Alnus), birch (Betula).

For more information on pollination by bees, check out our special article.